For example, Patent Literature 1 listed below describes one of hybrid electric vehicles including a motor generator, as another power source, other than an engine. In the hybrid electric vehicle disclosed in this Patent Literature 1, if operating conditions, such as, a vehicle speed, are fulfilled, driving the vehicle only by the motor generator, with the engine halted, improves fuel efficiency. A hybrid electric vehicle described in Patent Literature 2 listed below lowers a battery charge level immediately before starting the vehicle on an uphill incline to avoid battery overcharge when electricity is generated by a motor driven by large engine torque upon start on the uphill incline.
Patent Literature 3 listed below discloses one approach to address torque ripple such as cogging torque in a motor used in a hybrid electric vehicle. Therefore, Patent Literature 4 listed below describes the art to restrain and prevent vehicle vibration by controlling operating conditions of two motors when the vehicle vibration is due to cogging torque.